Symptoms caused by deterioration in brain functions include dementia such as an Alzheimer's disease. The number of Alzheimer's disease patients is approximately 2 million at present and is estimated to increase to 4 million until 2045. This is a very serious social problem to Japan facing an aging society. Although there are some therapeutic agents for the Alzheimer's disease, no fundamental therapeutic method thereof has been established up to date.
Although the causes of the Alzheimer's disease haven't been solved yet, an amyloid β-protein is closely related to the onset of the Alzheimer's disease. Thus it is believed that inhibition of synthesis and accumulation of the amyloid β (Aβ) is crucial in pursuit of achieving a complete cure. For this goal, therapeutic agents have been under development using various strategies that involve β and γ-secretase inhibitors, neutralizing antibodies against Aβ, activation of neprilysin functioning as an Aβ degrating enzyme, and the like, however satisfactory therapeutic agents have not been obtained yet.
The reason for this is that accumulation of Aβ in the brain has reached an advanced stage and brain neurons have died by the time the Alzheimer's disease is diagnosed, making it harder to treat the disease in this circumstance. Consequently, there is a need to develop a therapeutic agent and a method prophylactically functioning in the prevention of nerve cell death by prophylactically removing Aβ before the disappearance of nerve cells, or a therapeutic agent and a method for maintaining and restoring the number of nerve cells by proliferating and differentiating neural stem cells in the brain even if nerve cells are caused to die.
Patent Literature 1 describes that an antioxidant including any two or more kinds of carotenoids, ascorbic acids, and tocopherols exhibits anti-oxidation ability in vitro. It is considered that oxidation stress is involved in the disturbance of nerve cells.
Further, as an effect of docosahexaenoic acid (DHA), there are many reports describing that it is effective in preventing deterioration of brain functions in addition to lowering blood lipids, exerting antithrombotic activity, and the like. It is also reported that a combination of DHA and lutein has an inhibitory effect on deterioration of brain functions (Patent Literature 2). However, according to meta-analyses of intervention studies on DHA, the effectiveness of DHA is yet to be proved.
Non-Patent Literature 1 shows that DHA, arachidonic acid, and the like have a proliferation effect of neural stem cells.